1. Field of the Invention
The present invention relates to an image recording apparatus in which a single sheet (for example, a cut sheet) and an elongated sheet (for example, a fan-fold sheet) can selectively be used. More particularly, it relates to an image recording apparatus including a convey means having a specific feature.
2. Related Background Art
In the past, as an output apparatus for a personal computer or a facsimile, image recording apparatuses having a pin feed tractor have been used.
An example of such conventional image recording apparatuses will now be explained with reference to FIG. 9 showing a construction of the conventional image recording apparatus as a sectional view. As a convey means for a recording medium, there are provided a convey roller 450 and pinch rollers 451a, 451b urged against the convey roller to be followingly driven therewith, and a so-called U-turn path is formed so that the recording medium is conveyed around the convey roller 450. For example, when a cut sheet is used as the recording medium, the cut sheet is pinched between the convey roller 450 and the pinch rollers 451a, 451b to be conveyed by a friction generated between the sheet and the rollers.
On the other hand, a continuous sheet (referred to as "fan-fold sheet" hereinafter) is used as the recording medium, pin holes of the fan-fold sheet are engaged by pins 452a of a pin feed tractor (referred to merely as "tractor" hereinafter) 452, and the fan-fold sheet is pinched between the convey roller 450 and the pinch rollers 451a, 451b to be conveyed to a recording portion by a friction generated between the sheet and the rollers. In this case, there is no problem so long as a conveying amount of the sheet given by the tractor 452 is always the same as a conveying amount of the sheet given by the convey roller 450. However, in fact, since there is a difference in speed between the tractor and the convey roller due to the dimensional tolerance and environmental condition, the pinch rollers 451a, 451b are disengaged from the convey roller 450 so that a conveying force of the convey roller 450 is not transmitted to the fan-fold sheet.
After an image is recorded on the recording medium by a recording head 453 in response to image information, the recording medium is discharged by a sheet discharge roller 454 and a pinch roller 455 urged against the sheet discharge roller.
However, in the above-mentioned conventional technique, since the fan-fold sheet is also conveyed around the convey roller 450, even when the pinch rollers 451a, 451b are disengaged from the convey roller, the conveying force of the convey roller is transmitted to the fan-fold sheet, with the result that sheet jam sometimes occur due to the difference in speed between the tractor 452 and the convey roller 450.
For example, when relation St&lt;Sf is established assumed that a conveying speed of the tractor 452 is St and a conveying speed of the convey roller 450 is Sf, excessive tension acts on the fan-fold sheet between the tractor 452 and the convey roller 450, so that the sheet may be torn to cause the sheet jam. Further, even when a surface of the convey roller 450 is smooth-finished to decrease transmission of the conveying force from the convey roller to the fan-fold sheet, a conveying force between the convey roller 450 and the pinch rollers 451a, 451b required for conveying the cut sheet is decreased, thereby causing poor sheet conveyance (slip). Further, since the convey roller 450 is made of material such as rubber having relatively high frictional coefficient, a slipping condition of the surface of the roller is changed in accordance with the environmental condition. In particular, under a high temperature/high humidity condition, the coefficient of friction of the surface of the convey roller is greatly increased, so that, when the fan-fold sheet is conveyed around the convey roller, the conveying force acting on the fan-fold sheet cannot stably be reduced.
On the other hand, when relation St&gt;Sf is established, the fan-fold sheet is loosed between the tractor 452 and the convey roller 450, so that the pin holes of the fan-fold sheet are disengaged from the pins 452a of the tractor 452 to thereby cause the sheet jam.